Photocomposition is a specific form of photography. The basic principles of using the illumination of an incandescent lamp to project through a transparency, employing lenses for focusing, is also employed in most types of photocomposition machines.
As size change of the projected image became desirable in photocomposition, various approaches have been followed. One is simply to change the lenses. Some machines employ turret mounted lenses for this purpose.
The Addressograph-Multigraph Corporation, incorporated in the State of Delaware, U.S.A., has developed and marketed a photocomposition machine employing a variable size image projection accomplished by a variator lens shiftable along the optical axis of the projected image by means of a lens carriage. In order to accomplish uniform illumination of the image, an, iris diaphragm is employed using a laterally positioned cam with an iris actuating cam follower riding the cam.
Such cam and cam follower actuation of an iris diaphragm is a known technique in the photographic art. As long as the angle of the cam is small with respect to the direction of rotation of the cam follower, direct iris control by a laterally positioned cam will be operative.
However, in order to provide a photocomposition machine from about 41/2-point to 74-point character size, the pressure angle between a direct action follower arm and an actuating cam becomes so large that binding occurs. Pressure angle is defined as that angle (at any point) between the normal to the pitch curve of the cam and the instantaneous direction of the follower motion.